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    <title>Document</title>
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    <script>
        console.log(null == undefined); // true
        console.log(null === undefined); // false
        console.log(null == 0); // false
        console.log(1 - undefined); // NaN
        console.log(1 * undefined); // NaN
        console.log(1 / undefined); // NaN
        console.log(1 + undefined); // NaN
        console.log(typeof null); // object
        console.log(typeof []); // object
        console.log(String(undefined)); //undefined
        console.log(parseFloat(undefined)); // NaN
        console.log(parseInt(undefined)); // NaN
        console.log(parseFloat(true)); // NaN
        console.log(parseFloat('true')); // NaN
        console.log(parseFloat('3.14px')); //3.14
        console.log(parseFloat('x3.14px')); //NaN
        console.log(parseInt(true)); // NaN
        console.log(NaN == NaN); // false //NaN不和任何值相等包括它本身
        console.log('123' > '9'); //false 比较的是unicode而且是从左往右逐个数字进行比较,可能会出现错误的结果
    </script>
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